In the manufacture of semiconductor devices, photolithography is often used. In a typical arrangement, a projection optics box (POB) contains the optical elements that are used to reduce the image and form it on a photosensitive substrate (wafer). Typically, the POB includes an arrangement of one or more mirrors and one or more lenses that are constructed and positioned to accomplish the intended result.
Occasionally, actuators are provided for acting upon the optical elements. One type of actuator that is used in a POB optic system is a Lorentz actuator. One of the issues that has to be confronted when using such actuators is that the actuators typically have a coil and magnet construction and in vacuum applications, such as within the POB, the coil, as well as the magnet, can have a large outgassing rate due to material properties and/or the design of the actuators. Outgassing occurs when a material is placed into a vacuum (very low atmospheric pressure) environment, subjected to heat, and some of the material's constituents are volatilized (evaporated or “outgassed”). As is known, different materials exhibit outgassing characteristics, including outgassing rates, when these materials are exposed to the above described conditions. The magnet can be made out of a material with some porosity which in effect acts as a sponge in a vacuum and the windings of the coil are made out of round copper wire with insulation disposed therearound. Because of the arrangement of the windings, there is always space, containing air, between the windings. Moreover, if the coil would be encapsulated only with a plastic, the results would not be satisfactory due to the outgassing properties of plastics.
The actuator is thus required to operate in ultra high vacuum (UHV) and therefore, in the case of a Lorentz actuator, the coil and magnet thereof must be shielded from vacuum in such a way that they meet the outgassing specification. If the outgassing is not controlled or mitigated in such a UHV lithography system, outgases will render the UHV lithography system useless in a very short time.
What has heretofore not been available is an improved shield assembly and method of shielding the magnet and coil of an actuator, e.g., a Lorentz actuator, that is to be used in an UHV lithography system.